This invention relates to certain novel cholestane derivatives. In particular, this invention relates to novel cholestane derivatives of formula I which are useful in that they inhibit the activity of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCOA) and inhibit the formation of serum cholesterol. HMGCOA is an enzyme which controls the rate at which cholesterol is synthesized in mammalian liver (one of the two principal in vivo sources of serum cholesterol). Thus, the compounds of the instant invention, are adapted to inhibit sterol biosynthesis in individuals predisposed to familial type II hypercholesterolemia. The significance of such compounds is widly recognized, eg, Breslow et al. Biochem. et Biophys. Acta, 398, 10 (1975); Betteridge et al., Brit. Med. J., 4 500 (1975); and Brown et al., J. Biol. Chem., 249, 7306 (1974).